In digital image processing, an image registration serves in particular to establish a correlation between a plurality of images of a region of interest that is to be imaged in such a way that when the plurality of images are combined pixel by pixel, those pixels are combined with one another which relate to the same part of the region to be imaged. During the image registration of a first image relative to a second image, for example, the first image can be transformed in such a way that those pixels which relate to the same part of the region to be imaged are located at the same position in the first image and in the second image.
In digital subtraction angiography (DSA), an original image pair set containing one or more original image pairs is typically acquired of the same region of a patient that is to be imaged, wherein each original image pair of the original image pair set has a contrast agent image and a mask image. The contrast agent image is typically acquired while a contrast agent is present in the region to be imaged. The mask image is typically acquired while no or an extremely small amount of contrast agent is present in the region to be imaged. Via a pixel-by-pixel subtraction of the mask image from the contrast agent image it is possible for a structure of the region to be imaged in which contrast agent is present to be visualized more clearly with respect to an environment of the structure that is free of contrast agent.
In three-dimensional digital subtraction angiography (3D-DSA), a 3D subtraction angiography image is determined based on an original image pair set containing a plurality of original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image. In particular, the 3D subtraction angiography image can be determined via a technique known as dual volume reconstruction. Using said technique, a 3D mask image dataset can be determined based on the mask images and/or transformed mask images and a 3D contrast agent image dataset can be determined based on the contrast agent images. The 3D subtraction angiography image can be determined through combination of the 3D mask image dataset with the 3D contrast agent image dataset.
The mask image and the contrast agent image are acquired at different points in time. In the time interval lying therebetween, the position of the region to be imaged can change. Possible reasons for this are e.g. a movement of the patient, mechanical vibrations of the raw data acquisition apparatus and/or of the C-arm, fluctuations in an X-ray beam focus or pulsating blood vessels. This can lead to a misregistration of the mask image relative to the contrast agent image such that in a pixel-by-pixel combination, in particular a subtraction, of the mask image and the contrast agent image, those pixels which relate to the same part of the region to be imaged are not combined with one another. The misregistration can result in the quality of a subtraction angiography image which is determined based on the mask image and the contrast agent image being degraded due to artifacts, and consequently can lead to misdiagnoses. Typically, the degradation in image quality is particularly severe when metal, e.g. a coil inserted into an aneurysm, is present in the region to be imaged. In 3D-DSA, a type of artifact known as an eggshell artifact can be produced in this way. In particular in the case of eggshell artifacts it is often difficult to decide to what extent a circulation of blood in the aneurysm is suppressed due to the coil.